docs: fix outdated links (#10040)

www/apps/book/app/learn/customization/customize-admin/route/page.mdx

@@ -26,7 +26,7 @@ The UI Route can be shown in the sidebar or added as a nested page.
   items={[
     {
       text: "Brand Module",
-      link: "/customization/custom-features/module"
+      link: "/learn/customization/custom-features/module"
     },
   ]}
 />

www/apps/book/app/learn/customization/customize-admin/widget/page.mdx

@@ -18,7 +18,7 @@ This chapter covers how to show the brand of a product in the Medusa Admin using
   items={[
     {
       text: "Retrieve Brand of Product API Route",
-      link: "/customization/extend-models/query-linked-records"
+      link: "/learn/customization/extend-models/query-linked-records"
     }
   ]}
 />